A coherent optical communication system transmits information by modulating the phase, frequency or amplitude of an optical signal traveling down an optical fibre. At the receiving end of the fibre the information is recovered by interfering the arriving optical signal with a reference light beam produced by a local oscillator laser. For the two beams to combine effectively their polarisation states must be matched. Unfortunately the polarisation state of the received light may vary unpredictably with time owing to small disturbances to the optical fibre through which the signal has travelled. To avoid loss of transmitted information it is therefore necessary to transform the polarisation state of either the signal or the local oscillator so that their polarisation states are matched, and also to vary this transformation as the polarisation states of the signal and local oscillator change. A complete polarisation control system therefore requires transducers capable of altering the polarisation state of a light beam, and an algorithm designed to control the transducers so that polarisation matching can always be achieved. Most effort before now have concentrated on the transducers alone. Previous schemes have been based on fibre squeezers, fibre cranks and loops, and Faraday rotation, but none of the schemes proposed satisified all the requirements of coherent transmission systems.
One specical problem that has to be addressed by polarisation control schemes for optical communications is that of tracking a randomly varying polarisation state without encountering a range limit of the transducers. Cranks and loops can be arranged to provide endless control, although they are mechanicallly cumbersome and hence rather slow. More recently it has been proposed that three linearly birefringent elements and one circularly birefringent element provide endless control when cascaded together in series. A system using five squeezers to provide endless control has been demonstrated, although only four of these were required if either the input state or the output state were fixed.
Before setting out the present invention, it is useful to introduce the Poincare sphere as a tool for visualising transformations of polarisation states caused by birefringent elements.
Circularly symmetric single mode optical fibre is not strictly single mode, but supports two degenerate modes. If propagation along the fibre is in the Z direction then it is convenient to chose these two modes as those with the electric fields polarised in the X (horizontal) and Y (vertical) directions. Any propagation mode supported by the fibre may be represented as a sum of these two principle modes. In loose terms the electric field can be split into a horizontal component, Ex, and a vertical component, Ey, as shown in FIG. 1. The complete field is specified by the amplitude and phase of both Ex and Ey. The polarisation of the light in the fibre is determined by the relative phase and amplitude of Ex and Ey; it does not depend on the absolute phase of the signal or the total power in the signal. The polarisation state can therefore be determined by a single complex number .sigma. defined as EQU .sigma.=Ey/Ex (1)
where E.sub.x,E.sub.y are the complex amplitudes of the vertical and horizontal components. Two diagramatic representations of a polarisation state are useful, namely the polarisation ellipse and the Poincare sphere. The polarisation ellipse is the ellipse traced out by adding horizontal and vertical vectors varying with time as EQU Re(Ex exp(jwt)) and Re(Ey exp(jwt)) (2)
respectively. FIG. 2 shows a polarisation ellipse with major and minor axes a.sub.s and b.sub.s aligned at an angle .phi. to the horizontal and vertical axes. The azimuth .phi. and the degree of ellipicity .psi., defined as EQU .psi.=.+-.arctan (b.sub.s /a.sub.s), (3)
characterise the polarisation state completely. If .psi.=0 then the polarisation ellipse collapses to a line to give a linear polarisation state at an angle .phi. to the horizontal axis, whereas if .psi.=.+-..pi./4 then the left circular or right circular polarisation states are obtained.
The Poincare sphere, as shown in FIG. 3, is an alternative representation of polarisation states. A polarisation state S with azimuth .phi. and ellipicity .psi. is represented by a point on the unit sphere with longitude 2.phi. and latitude 2.psi.. All linear states lie on the equator, with the points H and V corresponding to horizontal and vertical states and points P and Q corresponding to states aligned at +.pi./4 and -.pi./4 to the horizontal fibre axis. Point L represents the left circular and point R right circular polarisations.
The Poincare sphere is particularly useful for describing the behaviour of birefringent elements. It can be shown that all birefringent elements transform polarisation states as rotations on the Poincare sphere. To illustrate this consider a fibre, shown in FIG. 4a, to which squeezing has been applied in the vertical direction. The stress induced in the fibre causes a different effective refractive index to be presented to the horizontal and vertical electric fields, so that these travel with a slightly different phase velocity. After traversing a length of this fibre the horizontal and vertical components appear with different phase. The fibre is said to be birefringent and, in general, it will change the polarisation state of light tramsitted through it. FIG. 5 shows how the polarisation state moves on the Poincare sphere. The birefringence between the horizontal and vertical polarisation states (H and V on the sphere) rotates any input state S about axis HV to an output state S'. The angle of rotation .theta. on the sphrer is equal to the phase difference introduced between the horizontal and vertical states on travelling through the fibre. In this example the horizontal and vertical states (H and V) are not affected by the rotation, so if the light is initially in either of these states it will remain so as it travels down the fibre.
In a similar way, squeezing applied at .+-..pi./4 to the fibre, shown in FIG. 4b, causes birefringence between the linear states aligned at .+-..pi./4 to the horizontal axis. This is represented on the sphere as a rotation about the axis QP, through an angle which equals the phase difference induced between these two states. In general, squeezing, the fibre at any angle .gamma. to the horizontal causes a rotation on the sphere about an axis lying in the equatorial plane with a longitude of 2.gamma..
Another case of interest is circular birefringence where the rotation is about the vertical axis LR. The Faraday effect gives rise to circular birefringence when a fibre is placed in a region of strong magnetic field.
Practical birefringent elements used to adjust or control polarisation separate broadly into two types. The first type includes the squeezing elements just discussed. These have the axes of rotation fixed and vary the amount of birefringence (the angle of rotation on the sphere) to adjust the output polarisation. The second type includes half-wave plates and quarter-wave plates used in bulk optics. These have the amount of birefringence fixed (by the thickness of the plate) and vary the rotation axis by rotating the element in space. Using a combination of fractional-wave elements (1/4,1/2,1/4) or a combination of squeezers (0, .pi./4, 0) it is possible to transform any polarisation state to any other state.
According to the present invention in a first main aspect there is provided apparatus for processing one or more optical signals to produce a desired polarisation transformation comprising at least one variable birefringent device adapted to provide a rotation of variable amount on the Poincar/e sphere about an axis, or effective axis, of rotation whcih itself may be varied in direction in a plane which passes through the origin of the Poincare sphere, and control means adapted to vary the amount of rotation on the Poincare sphere produced by the birefringent device, and to vary the direction of the axis or effective axis about which the rotation takes place, so as to achieve the desired polarisation transformation.
In preferred forms, the control means is arranged to vary the direction of the axis of rotation on the Poincare sphere in such a manner as to maintain below a chosen value the amount of birefringence introduced by the birefringent device to achieve the desired polarisation transformation, most preferably in such a manner as to minimise the amount of birefringence introduced by the birefringent device to achieve the desired polarisation transformation.
Preferably the control means is arranged to vary the direction of the axis of rotation on the Poincar/e sphere in such a manner as to prevent substantially the magnitude of the birefringence introduced by the device exceeding 2.pi. or a chosen miltiple of 2.pi.. In one preferred form, the control means is adapted to detect any inctrease of the magnitude of birefringence introduced by the device beyond 2.pi. or a chosen multiple of 2.pi., and upon such detection to rotate the axis of rotation of the device on the Poincare sphere through an angle of magnitude of .pi. or an odd multiple of .pi.. This detection may be achieved in a number of ways. In one form the control means is adapted to detect when a transformation has been introduced such that a rotation of the said axis of rotation has no effect on the relationship between the initial and final states of polarisation between which the transformation has taken place. In another form the variable birefringent device is calibrated to allow the control means to detect when a birefringence of magnitude of 2.pi. or a chosen multiple of 2.pi. has been introduced by the device.
In most practical embodiments, it will be arranged that the said variable birefringent device is capable of producing rotation on the Poincare sphere up to a finite limit, but is capable of endless rotation of the axis, or effective axis, of rotation by which the device effects change of polarisation state on the Poincare sphere.
In one application of the invention, the control means is adapted to produce a polarisation transformation from a fixed and known state of polarisation of circularly polarised light to a state of polarisation which varies with time, the birefringent device being capable of endless rotation of the direction of its axis of rotation on the Poincare sphere, in some arrangements, the birefringent device being capable of a maximum rotation on the Poincare sphere of .+-..pi..
In another more usual application of the invention, the control means is adapted to produce a polarisation transformation from a first state of polarisation which varies with time to a second state of polarisation which varies with time, the birefringent device being capable of endless rotation of the direction of its axis of rotation on the Poincare sphere, conveniently the birefringent device being capable of a maximum rotation on the Poincare sphere of .+-.2.pi..
In accordance with one particularly important feature of the invention, there are provided first and second variable birefringent devices, each adapted to provide a rotation of variable amount on the Poincare sphere about an axis, or effective axis, of rotation which itself may be varied in direction in a plane which passes through the origin of the Poincare sphere, means for assessing the desired polarisation transformation, and control means adapted to vary the birefringent devices in response to an output from the said assessing means, in which the control means is adapted to effect the required transformation in normal operation by varying one of said devices, and is adapted to detect when the said one device reaches a limit of birefringence, and thereafter to effect the required transformation by varying the previously non-operative device, while at the same time progressively reducing the birefringence introduced by the previously operative device.
In accordance with one particularly important application of the invention, the apparatus is arranged for adjusting the state of polarisation of one or both of two optical signals to achieve a chosen relationsip between the states of the signals, the apparatus including means for assessing the relationship between the polarisation states of the two signals after the said polarisation transformation, and the control means being adapted to vary the birefringent device in response to an output from the said assessing means.
In a particularly preferred form of this arrangement, the apparatus includes a second variable birefringent device adapted to provide a rotation of variable amount on a Poincare sphere about an axis of rotation which itself may be varied in direction in a plane which passes thropugh the origin of the Poincare sphere, in which the control means is adapted to effect the required transformation in normal operation by varying one of said devices to track changes in the states of polarisation of one or both of the optical signals. In accordance with a preferred feature, the control means is adapted to control the non-tracking device so as to reduce any birefringence being produced by the non-tracking device.
Another preferred feature of this form of the invention is that the control means may be adapted to detect when the tracking device reaches a limit of birefringence, and thereafter to effect the required transformation by varying the previously non-tracking device, which then becomes the actively tracking device, while at the same time progressively reducing the birefringence introduced by the previously tracking device, which has now become the non-tracking device, away from the said limit.
Preferably the control means is adapted to reduce the birefringence introduced by a non-tracking device until either that birefringence reaches zero or until the other, tracking, element reaches a limit of birefringence, whereupon in either eventuality the control means interchanges the functions of the two elements.
In some arrangements it is possible to operate the apparatus when each device is capable of producing a rotation on a Poincare sphere of a magnitude of slightly more than .pi. or a chosen multiple of .pi., and the control means is set to interchange the functions of the two elements if either element reaches a state where it introduces a rotation of magnitude of .pi. or a chosen multiple of .pi. on the Poincare sphere. However a more robust system is obtained when each device is capble of producing a rotation on a Poincare sphere of a magnitude of slightly more than 2.pi. or a chosen multiple of 2.pi., and the control means is set to interchange the functions of the two elements if either element reaches a state where it introduces a rotation of magnitude of 2.pi. or a chosen multiple of 2.pi. on the Ponicare sphere.
In some forms of the invention, it may be arranged that the control means repeatedly exchanges the tracking and non-tracking devices so as to maintain the birefringence of both devices below a chosen limit.
In general, in the description of the invention above, reference has been made to the axis of rotation on the Poincare sphere or to the effective axis. In many arrangements, the polarisation transformation can be represented as taking place about an axis on the Poincare sphere which is varied in direction. In other arrangements the same effect may be produced, but by a series of infinitesimal, or very small, rotations about two distinct axes giving rise to a net rotation about an effective axis lying substantially in the plane of the said two axes.
Thus in accordance with one form of the invention, it may be arranged that the birefringent device is adapted to provide a series of small rotations on a Poincare sphere about a first axis of rotation, and a series of small rotations about a second axis, and to alternate rapidly between rotations of the two series so as to give an effect equivalent to a rotation about a third, effective, axis, the control means being adapted to vary the relative amounts of the said small rotations of the said two series in such a manner as to vary the direction of the said effective axis. Conveniently, the said first and second axes may be orthogonal.
To summarise in general, there may be provided in accordance with the present invention a birefringent element in which both the amount of birefringence and the axis of rotation on the Poincare sphere can be varied. The element may provide linear birefringence at any angle in space so that rotation about any axis in the equitorial plane of the Poincare sphere is possible. Further, the angle in space may itself rotate without limit. For a fibre squeezer this would in principle be achieved by rotating the squeezers around the fibre, although this may not be possible in practice. The total amount of bvirefringence about any one axis may, however, be limited, this corresponding to a maximum squeezing pressure on the fibre.
Next, in accordance with a second main aspect of the present invention, there is provided a particular form of variable birefringent device, suitable for use in the appratus set out above. In this particularly preferred form, the or each variable birefringent device comprises a waveguide formed of electro-optic material, and input means for producing in the electro-optic material an electric field of variable strength and variable orientation, the control means being adapted to vary the amount of rotation on the Poincare sphere by varying the strength of the electric field in the electro-optic material, and to vary the direction of the axis of rotation on the Poincare sphere by varying the orientation of the electric field in the electro-optic material. Conveniently, the input means are arranged to apply to the electro-optic material two orthogonal electric fields both at right angles to the direction of propagation of light through the material, the control means being arranged to vary the strengths of the two orthogonal electric fields in such a manner as to vary the strength and orientation of the net electtric field in the material.
Preferably the electro-optic material has substantially the same refractive index in all directions perpendicular to the direction of propagation of light. However, the waveguide in the said electro-optic material may include a residual birefringence independent of any applied electric fields, and the input means may be arranged to apply a corrective voltage to counteract the residual birefringence, in addition to the application of the varying electric fields to achieve the required transformation on the Poincare sphere. Alternativelyy, the residual birefringence may be overcome by offsetting the direction of propagation of light slightly from the crystalographic axes of the electro-optic material.
It is particularly preferred that the electro-optic material is lithium niobate, for example the or each birefringent device may comprise a z-propagating waveguide diffused into a lithium niobate substrate.
Conveniently the input means may comprise at least three elongate electrodes arranged symemtrically over the waveguide, two outer electrodes being positioned one on either side of the waveguide and being arranged to provide a first electric field in the waveguide by a potential difference between the outer electrodes, and one or more central electrodes being arranged to provide a second electric field substantially at right angles to the first electric field in the electro-optic material, by means of a potential difference between the central electrode or electrodes, and the two outer electrodes.
Alternatively it may be arranged that the input means comprises a plurality of parallel electrodes extending generally along the length of the wave guide, but arranged to cross to and from across the waveguide repeatedly, the arrangement being such that in effect two orthogonal electric fields are distributed uniformly along the length of the waveguide, and can be varied by the control means.
Although the particular form of variable birefringent device discussed above has been set out in the context of the first aspect of the present invention, it is to be appreciated that such a variable birefringent device is versatile, and may be used for other purposes in the field of polarisation control.
In accordance with a first further aspect of the invention there may be provided apparatus for adjusting the state of polarisation of one or both of two optical signals to achieve a chosen relationship between the states of the signals, comprising first and second variable birefringent devices, each adapted to provide a rotation of variable amount on the Poincare sphere about an axis, or effective axis, of rotation which itself may be varied in direction in a plane which passes through the origin of the Poincare sphere, means for assessing the relationship between the polarisation states of the two optical signals after the transformation, and control means adapted to vary the birefringent devices in response to an output from the said assessing means, in which the control means is adapted to effect the required transformation in normal operation by varying one of said devices, and is adapted to detect when the said one device reaches a limit of birefringence, and thereafter to effect the required transformation by varying the previously non-operative device, while at the same time progressively reducing the birefringence introduced by the previously operative device.
In accordance with a second further aspect of the invention there may be provided apparatus for procesisng one or more optical signals to produce a desired polarisation transformation comprising at least one variable birefringent device adapted to provide a series of small rotations on a Poincare sphere about a first axis of rotation, and a series of small rotations about a second axis, and to alternate rapidly between the rotations of the two series so as to give an affect equivalent to a rotation about a third, effective, axis, and control means adapted to vary the cumulative amounts of the small rotations so as to vary the total rotation on the Poincare sphere, and to vary the relative amounts of the small rotations of the said two series so as to vary the direction of the said effective axis.
In accordance with a third further aspect of the invention there may be provided apparatus for processing one or more optical signals to produce a desired polarisation transformation comprising at least one variable birefringent device adapted to provide a rotation of variable amount on the Poincare sphere about an axis, or effective axis, of rotation which itself may be varied in direction, in a plane which passes through the origin of the Poincare sphere and control means adapted to vary the amount of rotation on the Poincare sphere produced by the birefringent device, and to vary the direction of the axis, or effective axis, about which the rotation takes place, so as to achieve the desired polarisation transformation, in which the control means is adapted to detect any increase of the magnitude of birefringence introduced by the device beyond 2.pi. or a chosen multiple of 2.pi., and upon such detection to rotate the axis, or effective axis, of rotation of the device on the Poincare sphere through an angle of mangitude of .pi. or an odd multiple of .pi..
In accordance with a fourth further aspect of the invention there may be provided apparatus for varying the state of polarisation of an optical signal by producing a desired polarisation transformation comprising, at least one variable birefringent device comprising a wavegiuide formed of elctro-optic material and an input means for producing one or more variable electric fields in the electro-optic material, and control means adapted to vary the strength and/or orientation of the electric fields so as to vary both the magnitude of the birefringence of the waveguide and the effective axis of the birefringence represented on the Poincare sphere, so as to achieve a desired polarisation transformation.